Titanium alloys and fiber composites are the benchmark classes of materials for fan and compressor blades in commercial airline engines. One reason for the materials being so broadly adopted is that regulations require an engine in commercial service to be capable of ingesting a medium-sized bird while allowing for continued operation or safe and orderly shutdown of that engine. Another reason is that the blades must resist cracking from nicks and dents caused by small debris such as sand and rain. Engines with titanium fan blades or certain reinforced fiber composite fan blades are the only ones that currently meet these criteria.
While titanium blades are relatively strong and light in weight, composite blades offer sufficient strength and a significant weight savings over titanium. However, composite blades do not scale well to smaller engine applications and the costs are several times those of already expensive titanium blades. Both titanium and fiber composite raw materials are also expensive to process. These blades often require expensive specialized equipment to process the material into an aerodynamic shape that maintains strength while keeping weight to a minimum. Further, due to their relatively low strain tolerance, composite blades require a greater thickness than otherwise equivalent metal blades to meet bird strike requirements. Greater blade thickness reduces fan efficiency and offsets a significant portion of weight savings from using composite materials.
Both solid and hollow blades made from titanium or titanium alloys, such as Ti-6Al-4V alloys, have been proven to offer sufficient hardness to resist erosion and foreign object damage and be certified for commercial service. However, diminishing returns are seen with these hollow blades as the cavities become smaller and approach the required thickness of the airfoil surfaces. Further, hollow blades are costly to produce using current techniques, particularly in the case of titanium, where expensive processes such as diffusion bonding are usually necessary to join two sheets together into a single hollow structure. Other less costly processes do not reliably produce the desired results without introducing unwanted stresses or weakening the blade in key areas.